Added prefix for source dir and a switch to modify relative link to source files to retain build number

Added prefix for source dir and a switch to modify relative link to source files to retain build number. The updated command would look like:
``` bash
diff_coverage.py xmlrunner/diff.patch --show-all --coverage-file=".coverage" --output-file="xmlrunner/html/diff_coverage.html" --root-package="_default_"  --link-prefix="source"  --reatin-build-no
```
###### How can we see source files in jenkins?

* If we retain old HTML reports then we would only be able to see the source file from jenkins from HTML report within a build
   * Jenkins only display source code for coverage and other such reports only for the latest build i.e. only the latest code
* If we are not to retain old HTML reports then there is no need to pass --reatin-build-no and we would only be able to see the source file from project level HTML report (i.e. without going into a specific build)